Oppel-Kundt Illusion and lateral optic flow manipulation in affecting perceived speed in approaching roundabouts: experiments with a driving simulator

Resolution A/RES/64/255 of the United Nations General Assembly proclaimed the period 2011–2020 as the “Decade of Action for Road Safety” worldwide, with the goal of stabilizing and then reducing the forecast level of road traffic fatalities around the world by increasing activities conducted at national, regional and global levels. Looking at the facts concerning road safety, as stated by the UN, one of the most important problems is vehicle speed. The aim of this work is to evaluate the potential influence of traffic-calming measures on drivers' speed. In particular, attention focuses on very simple, low-cost perceptual measures based on the Oppel-Kundt illusion and lateral optic flow manipulation. A driving simulator experiment was used to analyse three countermeasures, located along the approach to a roundabout in a rural area, against the control condition (no countermeasures, corresponding to the current configuration of the roundabout in question): evenly spaced guide posts, unevenly spaced guide posts, and optical speed bars. Results showed that both evenly and unevenly spaced guide posts significantly reduced drivers' speed with respect to the control condition. Mean speed reduction, measured between the entry and exit points of the analysed segment, was approximately 4.2%, which was significant in terms of reduction of risk. No difference was found between the treatments called ‘constant guide posts’ and ‘wide-to-thin guide posts’. The potential for reducing drivers' speed was only moderate in the case of optical speed bars. From the practical point of view, the proposed measures have the additional advantages of being easy to implement and not requiring regular maintenance, compared with other common measures based on pavement marking.